Ta2O5 as gate dielectric material for low-voltage organic thin-film transistors

In this paper we report the use of Ta₂O₅ as gate dielectric material for organic thin-film transistors. Ta₂O₅ has already attracted a lot of attention as insulating material for VLSI applications. We have deposited Ta₂O₅ thin-films with different thickness by means of electron-beam evaporation. Being a relatively low-temperature process, this method is particularly suitable for organic thin-film transistor fabrication on plastic substrates. Deposition and patterning are achieved in one step by the use of shadow masks. The dielectric can be evaporated on top of the semiconducting layer. In this way a large variety of structures can be realized. Poly(3-hexylthiophene) was used as semiconducting material in the transistor structure. Such transistors are operating at voltages smaller than −3 V. Having a high dielectric constant (ε_r=21), Ta₂O₅ facilitates the charge carrier accumulation in the transistor channel at much lower electrical fields. The properties of the dielectric material as well as the operation of the organic transistors with a Ta₂O₅ gate dielectric are discussed.     

Polyimide/polyvinyl alcohol bilayer gate insulator for low-voltage organic thin-film transistors

In this paper, we report the fabrication of a polyimide/polyvinyl alcohol (PVA) bilayer gate insulator for low-voltage organic thin-film transistors (TFTs). The introduction of a PVA layer to form a bilayer structure improves the dielectric and insulating properties of the gate insulator. Organic TFTs with 150 nm-thick polyimide and PVA gate insulators were inactive at low operation voltages below 5 V. Conversely, organic TFTs with 150 nm-thick polyimide/PVA bilayer gate insulators exhibited excellent device performances. Our results suggest that the introduction of a PVA layer with a high dielectric constant could be a simple and efficient way to improve the device performance of low-voltage organic TFTs.     

ZnO transparent thin-film transistors with HfO/sub 2//Ta/sub 2/O/sub 5/ stacking gate dielectrics

The performance improvement of ZnO thin-film transistors (TFTs) using HfO₂/Ta₂O₅ stacked gate dielectrics was demonstrated. The ZnO TFTs exhibited transistor behaviour over the range 0-10 V; the field effect mobility, subthreshold slope and on/off ratio were measured to be 1.3 cm² V^-1 s^-1, 0.5 V/decade and ~10⁶, respectively.     

Effect of gate insulator thickness on device performance of InGaZnO thin-film transistors

Top-contact thin-film transistors (TFTs) are fabricated in this work using atomic layer deposition (ALD) Al₂O₃ as the gate insulator and radio frequency sputtering InGaZnO (IGZO) as the channel layer so as to investigate the effect of Al₂O₃ thickness on the performance of IGZO-TFTs. The results show that TFT with 100-nm-thick Al₂O₃ (100 nm-Al₂O₃-TFT) exhibits the best electrical performance; specifically, field-effect mobility of 5 cm²/Vs, threshold voltage of 0.95 V, I_on/I_off ratio of 1.1×10⁷ and sub-threshold swing of 0.3 V/dec. The 100 nm-Al₂O₃-TFT also shows a substantially smaller threshold voltage shift of 1.1 V after a 10 V gate voltage is applied for 1 h, while the values for TFTs with an Al₂O₃ thickness of 220 and 280 nm are 1.84 and 2 V, respectively. The best performance of 100 nm-Al₂O₃-TFT can be attributed to the larger capacitance and the smaller amount of total trap centers possessed by a thinner insulator compared to the thicker ones.     

Low voltage copper phthalocyanine organic thin film transistors with a polymer layer as the gate insulator

Low voltage organic thin film transistors(OTFTs) were created using polymethyl-methacrylate-co g-lyciclyl-methacrylate(PMMA-GMA) as the gate dielectric.The OTFTs performed acceptably at supply voltages of about 10 V.From a densely packed copolymer brush,a leakage current as low as 2×10~(-8) A/cm~2 was obtained.From the measured capacitance-insulator frequency characteristics,a dielectric constant in the range 3.9-5.0 was obtained. By controlling the thickness of the gate dielectric,the threshold voltage ...     

基于聚合物绝缘栅的低电压有机薄膜晶体管

利用甲基丙烯酸甲酯-甲基丙烯酸环氧丙脂为栅绝缘层制备了酞箐铜有机薄膜晶体管,在电压为10V时器件具有较好的性能,栅绝缘层的漏电流密度低至2×10-8A/cm2 。测量其电容特性,该绝缘薄膜的介电常数介于3.9-5.0 。通过对绝缘层的减薄,阈值电压由 -3.5V 升至-2.0V,该酞箐铜有机薄膜晶体管可以在低电压下工作,其场效应迁移率为1.2×10-3 cm2/Vs 。     

Integration of a graphene ink as gate electrode for printed organic complementary thin-film transistors

We demonstrate a new electrode gate based on graphene ink for complementary printed organic metal oxide semiconductor (CMOS) technology on flexible plastic substrates. The goal is to replace the standard silver electrode gate. Devices made with graphene were enhanced and showed a high field-effect mobility of 3 cm² V⁻¹ s⁻¹ for P-type and 0.9 cm² V⁻¹ s⁻¹ for the N-type semiconductors. The improvement is attributed to the increase of the electrical capacitance of the organic dielectric (CYTOP) due to the graphene layer. A seven-stage ring oscillator was made with high oscillation frequencies of 2.1 kHz at 40 V corresponding to a delay/gate value of 34 μs. These performances are promising for use of low cost printed electronic applications.     

Organic thin-film field-effect transistors with MoO3/Al electrode and OTS/SiO2 bilayer gate insulator

An organic thin-film transistor (OTFTs) having OTS/SiO₂ bilayer gate insulator and MoO₃/Al electrode configuration between gate insulator and source–drain (S–D) electrodes has been investigated. Thermally grown SiO₂ layer is used as the OTFT gate dielectric and copper phthalocyanine (CuPc) for an active layer. We have found that using silane coupling agents, octadecyltrichlorosilane (OTS) on SiO₂, surface energy of SiO₂ gate dielectric is reduced; consequently, the device performance has been improved significantly. This OTS/SiO₂ bilayer gate insulator configuration increases the field-effect mobility, reduces the threshold voltage and improves the on/off ratios simultaneously. The device with MoO₃/Al electrode has similar source–drain current (I_DS) compared to the device with Au electrode at same gate voltage. Our results indicate that using double-layer of insulator and modified electrode is an effective way to improve OTFT performance.     

Low-voltage organic thin film transistors with hydrophobic aluminum nitride film as gate insulator

This paper reports on the low-voltage (<5 V) pentacene-based organic thin film transistors (OTFTs) with a hydrophobic aluminum nitride (AlN) gate-dielectric. In this work, a thin (about 50 nm), smooth (roughness about 0.18 nm) and low-leakage AlN gate dielectric is obtained and characterized. The AlN film is hydrophobic and the surface free energy is similar to the organic or the polymer films. The demonstrated AlN–OTFTs were operated at a low-voltage (3–5 V). A low-threshold voltage (−2 V) and an extremely low-subthreshold swing (∼170 mV/dec) were also obtained. Under low-voltage operating conditions, the on/off current ratio exceeded 10⁶, and the field effect mobility was mobility was 1.67 cm²/V s.     

Degradation behavior of Ta2O5 stacks and its dependence on the gate electrode

Response of 8 nm Ta₂O₅ stacks with Al and Au gate electrodes to voltage stress at room temperature and at 100 °C is investigated. Stress-induced leakage current (SILC) reveals significant gate dependence and distinct difference to SILC in SiO₂. The mechanisms for SILC generation and stress degradation are discussed. Unlike SiO₂, pre-existing traps and positive charge build-up are recognized as a key factor for generation of SILC in Ta₂O₅ stacks.     

Hydrogenated amorphous silicon thin-film transistor on plastic with an organic gate insulator

We developed a high-performance, hydrogenated amorphous silicon thin-film transistor (a-Si:H TFT) on plastic substrate using an organic gate insulator. The TFT with a silicon-nitride (SiN/sub x/) gate insulator exhibited a field-effect mobility of 0.3 cm/sup 2//Vs and a threshold voltage of 5 V. On the other hand, an a-Si:H TFT with an organic gate insulator of BCB (benzocyclobutene) has a field-effect mobility of 0.4 cm/sup 2//Vs and a threshold voltage of 0.7 V. The leakage currents through the gate insulator of an a-Si:H TFT with an organic gate insulator is about two orders of magnitude lower than that of an a-Si:H TFT with a SiN/sub x/ gate insulator.     

Performance and off-state current mechanisms of low-temperatureprocessed polysilicon thin-film transistors with liquid phase depositedSiO2 gate insulator

Polysilicon thin-film transistors (poly-Si TFT's) with liquid phase deposition (LPD) silicon dioxide (SiO₂) gate insulator were realized by low-temperature processes (<620°C). The physical, chemical, and electrical properties of the new dielectric layer were clarified. The low-temperature processed (LTP) poly-Si TFT's with W/L=200 μm/10 μm had an on-off current ratio of 4.95×10 ⁶ at V_D=5 V, a field effect mobility of 25.5 cm ²/V·s at V_D=0.1 V, a threshold voltage of 6.9 V, and a subthreshold swing of 1.28 V/decade at V_D=0.1 V. Effective passivation of defects by plasma hydrogenation can improve the characteristics of the devices. The off-state current (I_L) mechanisms of the LTP poly-Si TFT's were systematically compared and clarified. The I_L is divided into three regions; the I_L is attributable to a resistive current in region I (low gate bias), to pure thermal generation current in region II (low drain bias), and to Frenkel-Poole emission current in region III (high gate bias and drain bias)     

Morphology transformations of pentacene on an UV-patternable polymer insulator and their effect on performance of flexible organic thin-film transistors

This work presents a low temperature with high resolution capability UV-patternable polymer, i.e. mr-UVCur06, for use as a gate insulator in OTFTs, by investigating the morphology transformation of pentacene deposited on the mr-UVCur06. The device structure is polyethylene terephthalate (PET)/indium-tin oxide (ITO)/mr-UVCur06/pentacene/Au (source/drain). In addition to its solution-processable capability, mr-UVCur06 is directly patterned by UV light in a low-temperature process. UV/ozone post-treatment of the patternable mr-UVCur06 can illuminate organic contaminants from its surface and increases surface energy. Experimental results indicate that a high surface energy existing at the mr-UVCur06 surface has produced in a larger ratio of I_{thin film phase}/I_{triclinic bulk phase} in pentacene. Then, the distance of pentacene molecular crystal structure, which is arranged in the thin film phase, is shorter than that in triclinic bulk phase. The performance of pentacene-based OTFTs can be enhanced with few contaminants and a high surface energy on the UV-patternable gate insulator. By performing UV/ozone post-treatment on the mr-UVCur06 insulator surface for 60 s, the OTFTs demonstrate a mobility, on/off drain current ratio, and V_T of 0.34 cm²/V s, 5.5 × 10⁴, and 2.5 V, respectively. Furthermore, the low-temperature photopatternable polymer dielectric paves the way for a relatively easy and low-cost fabrication of an OTFT array without expensive and complicated photolithography and dry etching.     

Environmental effects on the electrical behavior of pentacene thin-film transistors with a poly(methyl methacrylate) gate insulator

The electrical properties of top-contact pentacene thin-film transistors (TFTs) with a poly(methyl methacrylate) (PMMA) gate dielectric were analyzed in air and vacuum environments. Compared to the vacuum case, the pentacene TFT in air exhibited lower drain currents and more pronounced shifts in the threshold voltage upon reversal of the gate voltage sweep direction, together with a decrease in the field-effect mobility. These characteristic variations were explained in terms of two distinctive actions of polar H₂O molecules in pentacene TFT. H₂O molecules were suggested to diffuse under the source and drain contacts and interrupt the charge injection into the pentacene film, whereas those that permeate at the pentacene/PMMA interface retard hole depletion in and around the TFT channel. The diffusion process was much slower than the permeation process. The degraded TFT characteristics in air could be recovered mostly by storing the device under vacuum, which suggests that the air instability of TFTs is due mainly to the physical adsorption of H₂O molecules within the pentacene film.     

Electrical properties of Ta/sub 2/O/sub 5/ gate dielectric on strained-Si

High dielectric constant (high-k) thin Ta/sub 2/O/sub 5/ films have been deposited on tensilely strained silicon (strained-Si) layers using a microwave plasma enhanced chemical vapour deposition technique at a low temperature. The deposited Ta/sub 2/O/sub 5/ films show good electrical properties as gate dielectrics and are suitable for microelectronic applications. The feasibility of integration of strained-Si and high-k dielectrics has been demonstrated.     

Low-voltage p-channel, n-channel and ambipolar organic thin-film transistors based on an ultrathin inorganic/polymer hybrid gate dielectric layer

A key issue in research into organic thin-film transistors (OTFTs) is low-voltage operation. In this study, we fabricated low-voltage operating (below 3V) p-channel, n-channel and ambipolar OTFTs based on pentacene or/and C₆₀ as the active layers, respectively, with an ultrathin AlO_X/poly(methyl methacrylate co glycidyl methacrylate) (P(MMA–GMA)) hybrid layer as the gate dielectric. Benefited from the enhanced crystallinity of C₆₀ layer and greatly reduced density of electron trapping states at the interface of channel/dielectric due to the insertion of ultrathin pentacene layer between C₆₀ and P(MMA–GMA), high electron mobility can be achieved in present pentacene/C₆₀ heterostructure based ambipolar OTFTs. The effect of the thickness of pentacene layer and the deposition sequence of pentacene and C₆₀ on the device performance of OTFTs was studied. The highest electron mobility of 3.50 cm²/V s and hole mobility of 0.25 cm²/V s were achieved in the ambipolar OTFT with a pentacene (3.0 nm)/C₆₀ (30 nm) heterostructure.     

Improving the electrical characteristics of MOS transistors with CeO2/La2O3 stacked gate dielectric

In this work, the electrical characteristics of MOSFETs with CeO₂/La₂O₃ gate dielectric stacks, where it was shown to have much lower amount of oxygen vacancy, were studied. We found that the negative threshold voltage shift in the La₂O₃-only transistors can be significantly suppressed. This improvement is attributed to the reduction of oxide charge density and to the dipole at the Si/La₂O₃ interface. Significant enhancement in channel mobility was also found for both NMOS and PMOS transistors. This latter improvement should be due to the silicon oxidation taking place at the La₂O₃/Si interface with the available of extra oxygen atoms from the CeO₂ layer. We further found that the subthreshold slopes for NMOS and PMOS transistors with 2.5 μm gate length were reduced to about 72 mV/dec, which are significantly smaller than those of transistors without using CeO₂ capping layer. This observation further confirms that the CeO₂ capping layer also affects the La₂O₃/Si interface properties.     

Below-one-volt organic thin-film transistors with large on/off current ratios

In many of the applications envisioned for organic thin-film transistors (TFTs), the electrical power will be supplied by small batteries or energy harvesters, which implies that it will be beneficial if the TFTs can be operated with voltages of 1 V or even below 1 V. At the same time, the TFTs should have large on/off current ratios, especially for applications in digital circuits and active matrices. Here we demonstrate p-channel and n-channel organic TFTs fabricated on a flexible plastic substrate that have a turn-on voltage of exactly 0 V, a subthreshold slope of 100 mV/decade, and an on/off current ratio of 2 × 10⁵ when operated with gate-source voltages between 0 and 0.7 V. Complementary inverters fabricated using these TFTs have a small-signal gain of 90 and a minimum noise margin of 79% at a supply voltage of 0.7 V. Complementary ring oscillators can be operated with supply voltages as small as 0.4 V.     

1 Volt organic transistors with mixed self-assembled monolayer/Al2O3 gate dielectrics

Control of the threshold voltage and the subthreshold swing is critical for low voltage transistor operation. In this contribution, organic field-effect transistors (OFETs) operating at 1 V using ultra-thin (∼4 nm), self-assembled monolayer (SAM) modified aluminium oxide layers as the gate dielectric are demonstrated. A solution-processed donor–acceptor semiconducting polymer poly(3,6-di(2-thien-5-yl)-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione)thieno[3,2-b]thiophene) (PDPP2TTT) is used as the active layer. It is shown that the threshold voltage of the fabricated transistors can be simply tuned by carefully controlling the composition of the applied SAM. The optimised OFETs display threshold voltages around 0 V, low subthreshold slopes (150 ± 5 mV/dec), operate with negligible hysteresis and show average saturated field-effect mobilities in excess of 0.1 cm²/V s at 1 V.     

Bar-coated high-performance organic thin-film transistors based on ultrathin PDFDT polymer with molecular weight independence

We report high-performance organic thin-film transistors (OTFTs) with an ultrathin active layer of difluorobenzothiadiazole-dithienosilole copolymer (PDFDT) form by using the wire bar-coating process. The top-gate/bottom contact (TG/BC) OTFTs based on bar-coated PDFDT polymer as channel material and poly(methyl methacrylate) (PMMA) as gate dielectric show a hole mobility of up to 2.2 cm² V⁻¹s⁻¹ with a current ON/OFF ratio (I_on/I_off) of 10⁴∼10⁵, with the mobility being two times larger than that of the spin-coated PDFDT based OTFTs. The higher mobility of the bar-coated PDFDT polymer films can be attributed to the well-organized fibril structures of the polymer chains. Importantly, two different molecular weight polymers (M_n = 23 and 34 kDa) were employed to conduct these experiments and both batches showed about the same performance, which mitigates the typical batch-to-batch variation in OTFT performance. Furthermore, we explored the operational stability of the bar-coated OTFTs in ambient air and nitrogen environments. The bias-stress and cycling tests between the ON/OFF states of the bar-coated devices showed high stability in both nitrogen and air. Conclusively, here we demonstrate that (i) a simple bar-coating process is a better method to control and obtain good polymer morphology in comparison to spin-coating, and (ii) the PDFDT polymer has great potential to provide good reproducibility and stability in large-area OTFT devices.